Heterogeneous chemical reactions are an important class of industrial processes in which the reactants are separate and to a large extent mutually insoluble. Many combinations of phases and dispersions are possible. Two of the more common arrangements are: liquid-liquid reactants with one liquid as a continuous phase having the second liquid dispersed throughout, generally in the form of drops; and, liquid-gas reactions with gas the continuous phase having drops of the liquid reactant dispersed throughout. In some cases the reactions are aided by the presence of a catalyst, either in homogeneous or heterogeneous form.
Two important examples of heterogeneous reactions are: nitration reactions, where typically an aromatic compound is dispersed throughout a solution of mixed acids; and desulfurizations, where a sulfur-laden hydrocarbon is sprayed into a hydrogen environment and passed over a catalyst bed. In the former example of nitration reactions, the mixed acids are usually nitric and sulfuric acid, with the sulfuric acid playing the role of a de facto catalyst, dissociating the nitric acid and forming a nitronium ion which comprises the reactive species.
An important example of an industrial nitration process is the nitration of benzene in a nitric-sulfuric acid mix to form mononitrobenzene (MNB), a precursor in the production of polyurethanes, among other important products. Another example, amongst many, is the nitration of toluene to dinitrotoluene, also a precursor in polyurethane production.
Reactions in heterogeneous systems generally take place near the interface between the reactants. For example in the case of liquid-liquid reactants, with one dispersed as drops throughout the second continuous reactant, it is well established that the actual reactions take place in the continuous phase just beyond the surface of the drop. This is also evident in the case of liquid-gas reactions such as normal combustion processes, where the fuel drop vaporizes into the surrounding oxygen rich mixture of gases, and the subsequent reaction occurs in the gas phase.
In the aforementioned simple liquid-gas combustion process, the drop of fuel continues to volatilize, the combustion products are swept away in the continuous phase, and the burning continues. Liquid-liquid reactions are somewhat more complex, as the reaction products formed near the interface must find their way into either of the reactants by diffusion or with the aid of other mass transfer phenomena. As the products of the reaction may remain in the reaction zone for a considerable time, the rate at which fresh reactants can be brought to the reaction zone is limited and the reaction slows.
The overall rate of reaction in liquid-liquid systems in particular can be increased by intensifying the two distinct steps of the process: dispersion, or break-up of drops, and coalescence, FIG. 1 shows these dispersion and coalescence steps schematically in an idealized way. In the dispersion step indicated by arrow 3, new, smaller drops 4 are continuously formed from larger drops 2 with the smaller drops having new fresh surface area between the reactants. In the coalescence step indicated by arrow 7, smaller drops 5 are brought together and join into larger drops 6 so that the reaction products can be mixed and withdrawn from the reaction zone.
The main forces that produce break-up or dispersion of drops in a flowing liquid are: local pressure fluctuations on account of turbulence; and shear forces adjacent to solid surfaces (that may either be moving or stationary). The main forces producing coalescence are: once again, pressure fluctuations due to natural turbulence that can propel the drops together; body forces such as gravity, which promote stratification and bringing together of the lighter component fluids, and fluid shear forces which can promote agglomeration or coalescence adjacent to a wall.
The role of dispersion in determining the overall reaction rate is well understood as being the creation of large amounts of fresh, new interfacial area between reactants (i.e. small drops). An appreciation of the importance of coalescence in determining the overall reaction rate can be gained by imagining the behavior of a drop with incremental steps in time as shown schematically in FIGS. 2a-2c. The situation illustrated is that of reaction products being much more soluble in the drop than in the continuous phase, as in the example generally of nitration of aromatic compounds in mixed acid.
FIG. 2(a) shows an idealized drop 2 freshly introduced into a second surrounding reactant 8 before any reaction has occurred. As the reaction is understood to take place in a region of the continuous phase just beyond the surface of the drop, after a short period, reaction products are formed (indicated by the darker band 10 around the drop 2) as shown in FIG. 2(b). If the drop were completely immobile, the reaction products would slowly diffuse into the drop, while the unreacted material would diffuse to the drop surface and thereby react further.
The role of coalescence is to accelerate the admixing of reacted with unreacted material by physically merging adjacent drops together. FIG. 2(c) shows the idealized situation after drops have coalesced, with reaction products 10 distributed throughout the unreacted material of a newly coalesced drop 6. Fresh unreacted material is now available at the drop surface to continue with the reaction.
It is appreciated that the description above is highly idealized, as the processes of dispersion and coalescence occur simultaneously in flowing liquid-liquid mixtures. New drops are continuously formed while old drops are merged by the combined actions of dispersion and coalescence, thereby sustaining the reaction. It becomes apparent however, that intensifying dispersion and coalescence phenomena can increase overall reaction rates.
Two conventional means for carrying out liquid-liquid reactions are in a so-called continuously stirred tank reactor 20 (CSTR), shown schematically in FIG. 3, or in a tubular, or pipe flow reactor 28 (PFR) shown in FIG. 4.
In the CSTR 20, a rotating impeller 22 imparts an overall circulation to the bulk fluid confined in a tank 24. While this can provide adequate mixing of miscible fluids, the situation with drops dispersed throughout a continuous fluid poses different issues. The greatest degree of dispersion occurs in the immediate vicinity of the rotating impeller as a result of the relatively high shear forces imparted by the moving surfaces. Although the bulk circulation is usually turbulent, drop dispersion rates are much lower in the bulk circulation than near the impeller. Turbulence in the bulk circulation however is responsible for most of the coalescence in the CSTR, and being relatively low, contributes to generally larger residence times being required to complete the reactions in a CSTR.
Pipe flow reactors (PFRs) 28, or in-line mixers as they are commonly called, have, as their name implies, the goal of mixing immiscible fluids together. Many examples of PFRs are in general industrial use. They generally comprise an enclosure 30 through reactants flow past insertable elements 32 which act to mix the flow. They often rely on a range of insertable elements 32 for use in different process and fluid conditions. Although certain specific types of elements can provide a modest degree of dispersion for immiscible drops, no particular amount of coalescence beyond that provided by the turbulent flow is achieved. Nevertheless this type mixer has been used as a reactor for nitrating benzene as described in European Patent Specification EP 0779270 B1 assigned to Mitsui Chemicals, Inc. The Mitsui patent describes nitration experiments with this type of reactor, and cites results that show high byproduct formation for instances having acceptable conversion rates of the incoming nitric acid. Conversely, the results showed low byproduct formation occurred at unacceptably low rates of nitric conversion.
Perhaps the first commercially successful reactor to deliberately use highly intensified dispersion zones is described in U.S. Pat. No. 4,994,242. The so-called jet-impingement reactor (assigned to Noram Engineering and Constructors Ltd.) uses a set of baffles, either flat, cylindrical or spherical having a series of holes allowing the passage of fluid. The intensified dispersion is achieved by high rates of shear generated in the flow as it passes adjacent to the sharp edge of a hole through the baffle. The holes in the adjacent baffles are slightly staggered in a lateral direction to avoid channeling through aligned holes, The high shear rates near the edge of the hole generate a high degree of dispersion as already mentioned, followed downstream by turbulent shear layers which merge into a turbulent jet. A certain degree of coalescence occurs in the jet downstream of a hole owing to the highly turbulent nature of the flow, but no other means are provided to intensify coalescence. In practice, a certain length of coalescing zone (usually a length of pipe) follows a set of baffles, typically 3-6 times the diameter of the baffles. Coalescence in this zone is generally low and is simply due to natural turbulence.
A reactor similar in design to the jet-impingement reactor is described in U.S. Pat. No. 6,506,949 issued Jan. 14, 2003 and assigned to Dow Global Technologies Inc. This reactor also uses a set of baffles with holes for drop dispersion, followed by sections of straight pipe to allow drop coalescence. A key feature distinguishing the Dow reactor from the Noram reactor is that the Dow design requires the reactor to be horizontal, whereas the Noram reactor can be arranged either horizontally or vertically. The baffle holes in the Dow reactor are located in the bottom part of the baffles. The claimed benefit of this arrangement when used for nitrating benzene in mixed acids is that, passing into a coalescence zone of straight pipe following a baffle, the benzene-MNB drops, being lighter than the surrounding mixed acid, will rise upward and coalesce in the upper portion of the pipe. As gravity is a relatively weak body force, a considerable length of pipe is needed to produce any significant coalescence (a most preferable coalescence zone length of 120 times the pipe diameter is cited in the Dow patent). This requirement leads to impractical reactor lengths and very long residence times, generally undesirable features.